Two-wire layered heater system

ABSTRACT

A heater system is provided with a layered heater in communication with a two-wire controller, wherein a resistive layer of the layered heater is both a heater element and a temperature sensor. The two-wire controller thus determines temperature of the layered heater using the resistance of the resistive layer and controls heater temperature through a power source. Furthermore, a heater system using a layered heater in communication with a two-wire controller for a specific application of a hot runner nozzle in an injection molding system is provided by the present invention.

FIELD OF THE INVENTION

The present invention relates generally to electrical heaters andcontrollers and more particularly to temperature sensing for layeredheaters.

BACKGROUND OF THE INVENTION

Layered heaters are typically used in applications where space islimited, when heat output needs vary across a surface, or in ultra-cleanor aggressive chemical applications. A layered heater generallycomprises layers of different materials, namely, a dielectric and aresistive material, which are applied to a substrate. The dielectricmaterial is applied first to the substrate and provides electricalisolation between the substrate and the resistive material and alsominimizes current leakage during operation. The resistive material isapplied to the dielectric material in a predetermined pattern andprovides a resistive heater circuit. The layered heater also includesleads that connect the resistive heater circuit to a heater controllerand an over-mold material that protects the lead-to-resistive circuitinterface. Accordingly, layered heaters are highly customizable for avariety of heating applications.

Layered heaters may be “thick” film, “thin” film, or “thermallysprayed,” among others, wherein the primary difference between thesetypes of layered heaters is the method in which the layers are formed.For example, the layers for thick film heaters are typically formedusing processes such as screen printing, decal application, or filmprinting heads, among others. The layers for thin film heaters aretypically formed using deposition processes such as ion plating,sputtering, chemical vapor deposition (CVD), and physical vapordeposition (PVD), among others. Yet another process distinct from thinand thick film techniques is thermal spraying, which may include by wayof example flame spraying, plasma spraying, wire arc spraying, and HVOF(High Velocity Oxygen Fuel), among others.

Known systems that employ layered heaters typically include a separatetemperature sensor, which is connected to the controller through anotherset of electrical leads in addition to the set of leads for theresistive heater circuit. The temperature sensor is often a thermocouplethat is placed somewhere near the film heater and/or the process inorder to provide the controller with temperature feedback for heatercontrol. However, the thermocouple is relatively bulky, requiresadditional electrical leads, and fails relatively frequently.Alternately, an RTD (resistance temperature detector) may beincorporated within the layered heater as a separate layer in order toobtain more accurate temperature readings and to reduce the amount ofspace required as compared with a conventional thermocouple.Unfortunately, the RTD also communicates with the controller through anadditional set of electrical leads. For systems that employ a largenumber of temperature sensors, the number of associated electrical leadsfor each sensor is substantial and results in added bulk and complexityto the overall heater system.

For example, one such application where electrical leads add bulk andcomplexity to a heater system is with injection molding systems.Injection molding systems, and more specifically hot runner systems,often include a large number of nozzles for higher cavitation molding,where multiple parts are molded in a single cycle, or shot. The nozzlesare often heated to improve resin flow, and thus for each nozzle in thesystem, an associated set of electrical leads for a nozzle heater and aset of electrical leads for at least one temperature sensor (e.g.,thermocouple) placed near the heater and/or the process must be routedfrom a control system to each nozzle. The routing of electrical leads istypically accomplished using an umbilical that runs from the controlsystem to a hot runner mold system. Further, wiring channels aretypically milled into plates of the mold system to route the leads toeach nozzle, and therefore, an increased number of electrical leads addscost and complexity to the hot runner mold system and adds bulk to theoverall injection molding system.

SUMMARY OF THE INVENTION

In one preferred form, the present invention provides a heater systemcomprising a thick film heater and a two-wire controller. The thick filmheater defines a substrate, a dielectric layer disposed on thesubstrate, and a resistive layer disposed on the dielectric layer,wherein the resistive layer has sufficient temperature coefficient ofresistance characteristics such that the resistive layer is a heaterelement and a temperature sensor. Further, a protective layer isdisposed over the resistive layer and the two-wire controller determinestemperature of the thick film heater using the resistance of theresistive layer and controls heater temperature accordingly.

In another form, a layered heater is provided that comprises at leastone resistive layer, wherein the resistive layer has sufficienttemperature coefficient of resistance characteristics such that theresistive layer is a heater element and a temperature sensor. Thelayered heater further comprises a two-wire controller connected to theresistive layer, wherein the two-wire controller determines temperatureof the layered heater using the resistance of the resistive layer andcontrols heater temperature accordingly. In the various forms of theinvention, the layered heater is a thick film heater, a thin filmheater, a thermally sprayed heater, and a sol-gel heater.

In yet another form, a hot runner nozzle heater system is provided thatcomprises at least one runner nozzle and at least one resistive layerdisposed proximate the runner nozzle, wherein the resistive layer hassufficient temperature coefficient of resistance characteristics suchthat the resistive layer is a heater element and a temperature sensor.The heater system further comprises a two-wire controller connected tothe resistive layer, wherein the two-wire controller determinestemperature of the heater system using the resistance of the resistivelayer and controls heater system temperature accordingly.

Additionally, the present invention provides a heater system for usewith an existing temperature controller having at least one temperaturesensor input and a power output. The invention is an improvement thatcomprises at least one layered heater having at least one resistivelayer, wherein the resistive layer has sufficient temperaturecoefficient of resistance characteristics such that the resistive layeris a heater element and a temperature sensor. The improvement furthercomprises at least one two-wire module connected to the layered heaterand to the temperature controller, wherein the two-wire moduledetermines temperature of the layered heater using the resistance of theresistive layer and transmits the temperature of the layered heater tothe temperature controller input, and the temperature controllertransmits the power output to the two-wire module.

In still another form, a heater system is provided that comprises alayered heater having at least one resistive layer, wherein theresistive layer has sufficient temperature coefficient of resistancecharacteristics such that the resistive layer is a heater element and atemperature sensor. The heater system further comprises an electricallead connected to the resistive layer and a controller connected to theresistive layer through the electrical lead, wherein the controllerdetermines temperature of the layered heater using the resistance of theresistive layer and controls heater temperature accordingly.Additionally, a common return device is connected to the layered heaterand a power source is connected to the controller, wherein the commonreturn device provides an electrical return to the controller from thelayered heater such that only a single wire is required for operation ofthe heater system.

According to a method of the present invention, operation of a layeredheater is provided that comprises the steps of supplying power to theheater through a set of leads to a resistive element of the layeredheater and calculating the temperature of the resistive element using atwo-wire controller in communication with the layered heater through theset of leads, wherein the resistive element is a heater element and atemperature sensor. In another form, the method is used to operate alayered heater in conjunction with a hot runner nozzle.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a block diagram of a heater system in accordance with theprinciples of the present invention;

FIG. 2 is an enlarged cross-sectional view of a layered heater inaccordance with the principles of the present invention;

FIG. 3 a is an enlarged cross-sectional view of a layered heatercomprising a resistive layer and a protective layer in accordance withthe principles of the present invention;

FIG. 3 b is an enlarged cross-sectional view of a layered heatercomprising only a resistive layer in accordance with the principles ofthe present invention;

FIG. 4 a is a plan view of a resistive layer pattern constructed inaccordance with the teachings of the present invention;

FIG. 4 b is a plan view of a second resistive layer pattern constructedin accordance with the principles of the present invention;

FIG. 4 c is a perspective view of a third resistive layer patternconstructed in accordance with the principles of the present invention;

FIG. 5 is a block diagram illustrating a two-wire control system inaccordance with the principles of the present invention;

FIG. 6 is a simplified electrical schematic of a two-wire control systemconstructed in accordance with the teachings of the present invention;

FIG. 7 is a detailed electrical schematic of a two-wire control systemconstructed in accordance with the teachings of the present invention;

FIG. 8 is a perspective view of a high cavitation mold for an injectionmolding system having a heater system with hot runner nozzles andconstructed in accordance with the teachings of the present invention;

FIG. 9 is a side view of a hot runner nozzle heater system constructedin accordance with the teachings of the present invention;

FIG. 10 is a side cross-sectional view of the hot runner nozzle heatersystem, taken along line A—A of FIG. 9, in accordance with theprinciples of the present invention;

FIG. 11 is a side cross-sectional view of an alternate embodiment of thehot runner nozzle heater system constructed in accordance with theteachings of the present invention;

FIG. 12 is a schematic diagram of a modular heater system for retrofitinto existing systems in accordance with the principles of the presentinvention; and

FIG. 13 is a block diagram of a heater system using a single wire inaccordance with the principles of the present invention.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

Referring to FIG. 1, a simplified heater system in block diagram formatin accordance with one form of the present invention is illustrated andgenerally indicated by reference numeral 10. The heater system 10comprises a layered heater 12, a two-wire controller 14, which ispreferably microprocessor based, and a power source 16 within orconnected to the two-wire controller 14. The layered heater 12 isconnected to the two-wire controller 14 as shown through a single set ofelectrical leads 18. Power is provided to the layered heater 12 throughthe electrical leads 18, and temperature information of the layeredheater 12 is provided on command to the two-wire controller 14 throughthe same set of electrical leads 18. More specifically, the two-wirecontroller 14 determines the temperature of the layered heater 12 basedon a calculated resistance, one technique of which is described ingreater detail below. The two-wire controller 14 then sends signals tothe power source 16 to control the temperature of the layered heater 12accordingly. Therefore, only a single set of electrical leads 18 isrequired rather than one set for the heater and one set for atemperature sensor.

Referring now to FIG. 2, in one form the layered heater 12 comprises anumber of layers disposed on a substrate 20, wherein the substrate 20may be a separate element disposed proximate the part or device to beheated, or the part or device itself. As shown, the layers preferablycomprise a dielectric layer 22, a resistive layer 24, and a protectivelayer 26. The dielectric layer 22 provides electrical isolation betweenthe substrate 20 and the resistive layer 24 and is disposed on thesubstrate 20 in a thickness commensurate with the power output of thelayered heater 12. The resistive layer 24 is disposed on the dielectriclayer 22 and provides two primary functions in accordance with thepresent invention. First, the resistive layer 24 is a resistive heatercircuit for the layered heater 12, thereby providing the heat to thesubstrate 20. Second, the resistive layer 24 is also a temperaturesensor, wherein the resistance of the resistive layer 24 is used todetermine the temperature of the layered heater 12 as described ingreater detail below. The protective layer 26 is preferably aninsulator, however other materials such as a conductive material mayalso be employed according to the requirements of a specific heatingapplication while remaining within the scope of the present invention.

As further shown, terminal pads 28 are disposed on the dielectric layer22 and are in contact with the resistive layer 24. Accordingly,electrical leads 30 are in contact with the terminal pads 28 and connectthe resistive layer 24 to the two-wire controller 14 (not shown) forpower input and for transmission of heater temperature information tothe two-wire controller 14. Further, the protective layer 26 is disposedover the resistive layer 24 and is preferably a dielectric material forelectrical isolation and protection of the resistive layer 24 from theoperating environment. Since the resistive layer 24 functions as both aheating element and a temperature sensor, only one set of electricalleads 30, (e.g., two wires), are required for the heater system 10,rather than one set for the layered heater 12 and another set for aseparate temperature sensor. Thus, the number of electrical leads forany given heater system is reduced by 50% through the use of the heatersystem 10 according to the present invention. Additionally, since theentire resistive layer 24 is a temperature sensor in addition to aheater element, temperature is sensed throughout the entire heaterelement rather than at a single point as with many conventionaltemperature sensors such as a thermocouple.

In another form of the present invention as shown in FIG. 3 a, theresistive layer 24 is disposed on the substrate 20 in the case where thesubstrate 20 is not conductive and electrical isolation is not requiredthrough a separate dielectric layer. As shown, the protective layer 26is disposed over the resistive layer 24 as previously described. In yetanother form as shown in FIG. 3 b, the resistive layer 24 is disposed onthe substrate 20 with no dielectric layer 22 and no protective layer 26.Accordingly, the heater system 10 of the present invention is operablewith at least one layer, namely, the resistive layer 24, wherein theresistive layer 24 is both a heating element and a temperature sensor.Other combinations of functional layers not illustrated herein may alsobe employed according to specific application requirements whileremaining within the scope of the present invention.

Generally, the layered heater 12 is configured for operation with anynumber of devices that require heating, one of which is hot runnernozzles for injection molding systems as described in greater detailbelow. Furthermore, the layered heater 12 is preferably a thick filmheater that is fabricated using a film printing head in one form of thepresent invention. Fabrication of the layers using this thick filmprocess is shown and described in U.S. Pat. No. 5,973,296, which iscommonly assigned with the present application and the contents of whichare incorporated herein by reference in their entirety. Additional thickfilm processes may include, by way of example, screen printing,spraying, rolling, and transfer printing, among others.

However, in another form, the layered heater 12 is a thin film heater,wherein the layers are formed using thin film processes such as ionplating, sputtering, chemical vapor deposition (CVD), and physical vapordeposition (PVD), among others. Thin film processes such as thosedisclosed in U.S. Pat. Nos. 6,305,923, 6,341,954, and 6,575,729, whichare incorporated herein by reference in their entirety, may be employedwith the heater system 10 as described herein while remaining within thescope of the present invention. In yet another form, the layered heater12 is a thermal sprayed heater, wherein the layers are formed usingthermal spraying processes such as flame spraying, plasma spraying, wirearc spraying, and HVOF (High Velocity Oxygen Fuel), among others. Instill another form, the layered heater 12 is a “sol-gel” heater, whereinthe layers are formed using sol-gel materials. Generally, the sol-gellayers are formed using processes such as dipping, spinning, orpainting, among others. Thus, as used herein, the term “layered heater”should be construed to include heaters that comprise at least onefunctional layer (e.g., resistive layer 24 only, resistive layer 24 andprotective layer 26, dielectric layer 22 and resistive layer 24 andprotective layer 26, among others), wherein the layer is formed throughapplication or accumulation of a material to a substrate or anotherlayer using processes associated with thick film, thin film, thermalspraying, or sol-gel, among others. These processes are also referred toas “layered processes” or “layered heater processes.”

In order for the resistive layer 24 to serve both the function of atemperature sensor in addition to a heater element, the resistive layer24 is preferably a material having a relatively high temperaturecoefficient of resistance (TCR). As the resistance of metals increaseswith temperature, the resistance at any temperature t (° C.) is:R=R ₀(1+αt)  (Equation 1)

where: R₀ is the resistance at some reference temperature (often 0° C.)and α is the temperature coefficient of resistance (TCR). Thus, todetermine the temperature of the heater, a resistance of the heater iscalculated by the two-wire controller 14 as described in greater detailbelow. In one form, the voltage across and the current through theheater is measured using the two-wire controller 14, and a resistance iscalculated based on Ohm's law. Using Equation 1, or similar equationsknown to those skilled in the art of temperature measurement usingResistance Temperature Detectors (RTDs), and the known TCR, temperatureof the resistive layer 24 is then calculated and used for heatercontrol.

Therefore, in one form of the present invention, a relatively high TCRis preferred such that a small temperature change results in a largeresistance change. Therefore, formulations that include materials suchas platinum (TCR=0.0039 Ω/Ω/° C.), nickel (TCR=0.0041 Ω/Ω/° C.), orcopper (TCR=0.0039 Ω/Ω/° C.), and alloys thereof, are preferred for theresistive layer 24.

However, in other forms of the present invention, a material for theresistive layer 24 need not necessarily have a high TCR. For example, anegative TCR material, or a material having a non-linear TCR, would alsofall within the scope of the present invention, as long as the TCR ispredictable. If the TCR of a given material is known, if it can bemeasured with the necessary accuracy, and if it is repeatable orpredictable, then the material could be used to determine temperature ofthe heater system 10. Such a TCR, including the relatively high TCRmaterials as described, are hereinafter referred to as having sufficientTCR characteristics. Accordingly, the materials described herein andtheir related high TCRs should not be construed as limiting the scope ofthe present invention. The relatively high TCR as described herein arepreferred in one form of the present invention.

As another sufficient TCR characteristic, the material used for theresistive layer 24 must not exhibit excessive “drift,” which is atendency of many resistive elements to change characteristics, such asbulk resistivity or TCR, over time. Therefore, the material for theresistive layer 24 is preferably stable or predictable in terms ofdrift, however, the drift can be compensated for over time throughcalibration of the two-wire controller 14 that is described in greaterdetail below. Additionally, drift can be reduced or eliminated through“burn-in” of the heater to induce any resistance shift that would occurover time. Accordingly, the resistive layer 24 is preferably a materialthat has a relatively high temperature coefficient of resistance andthat is stable in terms of drift. However, if the drift is predictable,the material may be used for the resistive layer while remaining withinthe scope of the present invention.

In one form of the present invention, the resistive layer 24 is formedby printing a resistive material on the dielectric layer 22 aspreviously set forth. More specifically, two (2) resistive materialswere tested for use in the present invention, RI1 and RI2, wherein theTCR of RI1 was between approximately 0.0008 Ω/Ω/° C. and approximately0.0016 Ω/Ω/° C., and the TCR of RI2 was between approximately 0.0026Ω/Ω/° C. and approximately 0.0040 Ω/Ω/° C. Additionally, temperaturedrift was tested for RI1 and RI2, at various temperatures, and the driftvaried from approximately 3% for RI1 to approximately 10% for RI2. Witha “burn-in” as previously described, the drift was shown to have beenreduced to approximately 2% for RI1 to approximately 4% for RI2. Thematerials for the resistive layer 24 and their respective values for TCRand temperature drift as described herein are exemplary in nature andshould not be construed as limiting the scope of the present invention.Any resistive material having sufficient TCR characteristics aspreviously set forth can be utilized for the resistive layer 24 whileremaining within the scope of the present invention.

Since a plurality of layered heaters having temperature sensingcapabilities are employed according to the present invention, thetwo-wire controller 14 must be provided with certain information aboutthe heaters, and more specifically the resistive layers 24, in order toproperly calibrate the overall heater system. Parameters that arenecessary for such calibration include the cold resistance, thetemperature at which the cold resistance value was measured, and certainTCR characteristics (TCR at a temperature and/or over a temperaturerange) in order to determine heater temperature from heater resistancecalculations. Preferably, the system automatically calculates the coldresistance of each layered heater 12 based on the measured voltage andcurrent using the two-wire controller 14 as described in greater detailbelow. Additionally, the TCR characteristics for each layered heater 12must be entered into the system, e.g. the two-wire controller 14, usingmanual and/or electronic methods. Such values may be enteredindividually or as a single value for all layered heaters 12 dependingon, for example, whether or not the material for the resistive layer 24came from a common manufacturing lot. Regardless, the calibration data,namely, the cold resistance, cold resistance temperature, and TCR ofeach layered heater 12 is preferably entered into the two-wirecontroller 14 for more accurate and controlled operation of the heatersystem 10.

A variety of methods of providing the TCR characteristics and coldresistance data of each layered heater 12 to the two-wire controller 14may be employed while remaining within the scope of the presentinvention. For example, each layered heater 12 may include a bar-codedtag that would be scanned by an operator to download the cold resistancedata and TCR characteristics to the two-wire controller 14. Alternately,a smart card chip or other electronic means may be attached to eachlayered heater 12, which would similarly be scanned by an operator todownload the calibration data to the two-wire controller 14. In yetanother form, the calibration data may be downloaded to the two-wirecontroller 14 via the Internet, for example, through a supplier website.Alternately, the TCR characteristics and cold resistance data may bepre-programmed into the two-wire controller 14.

In addition to calibration for resistance data and TCR, compensation forthe resistance of electrical leads 30 is also provided by the heatersystem 10 according to the present invention. Since the electrical leads30 add resistance to the circuit, temperature errors would likely resultif no compensation for the increase in resistance were provided.Additionally, the materials used for the electrical leads 30 may have aTCR higher than that of the resistive layer 24, which results in theportion of the electrical leads 30 that are exposed to highertemperatures contributing more resistance. Therefore, the two-wirecontroller 14 also provides for calibration of lead wire resistance.

The two-wire controller 14 is preferably designed with temperaturecalibration capabilities, which further reduces long term temperatureerrors due to drift. One method of temperature calibration isaccomplished by using one or more pre-existing thermocouples, or otherpre-existing temperature sensors, to ascertain both the temperature andthe stability of the temperature. The temperature data from thethermocouples is then transmitted to the two-wire controller 14 for theresistance calculations. Further, changes in the measured coldresistance of the layered heater 12 may be used to calculate new TCRvalues as appropriate. In another form for temperature calibration, thetwo-wire controller 14 preferably comprises a calibration offset featurethat provides for input of a temperature offset parameter. Such anoffset is desirable when the location of the layered heater 12 is somedistance away from the optimum location for sensing temperature. Thus,the temperature offset parameter may be used such that the heater system10 provides a temperature that more closely represents the actualtemperature at the optimum location.

Turning now to the construction of the layered heater 12 as shown inFIGS. 4 a–4 c, the resistive layer 24 is preferably disposed on thedielectric layer 22 in a pattern 40 that results in a desiredtemperature profile for the given substrate or element being heated.FIG. 4 a shows a resistive layer 24 a in a rectangular pattern 40 abased on the rectangular profile of the substrate 20 a. FIG. 4 b shows aresistive layer 24 b in a circular pattern 40 b based on the circularprofile of the substrate 20 b. FIG. 4 c shows a resistive layer 24 c ina spiral pattern 40 c based on a cylindrical shape of the substrate 20c. Additionally, the width “W” and/or pitch “P” of the patterns 40 a–cmay also be altered according to the specific heating requirements ofthe heater system. Therefore, the pattern of the resistive layer 24 a ispreferably customized for each application of the heater system 10. Thepatterns illustrated herein are exemplary only and are not intended tolimit the scope of the present invention.

The layered heater 12, including each of the layers and the terminalpads 28 may also be constructed in accordance with U.S. Pat. Nos.6,410,894, 6,222,166, 6,037,574, 5,973,296, and 5,714,738, which arecommonly assigned with the present invention and the contents of whichare incorporated herein in their entirety, while remaining within thescope of the present invention. Accordingly, additional specificity withregard to further materials, manufacturing techniques, and constructionapproaches are not included herein for purposes of clarity and referenceis thus made to the patents incorporated by reference herein for suchadditional information.

Two-Wire Controller (14)

One form of the two-wire controller 14 is illustrated in block diagramformat in FIG. 5. As shown, the two-wire controller 14 generallycomprises a power source 50, a voltage and current measurement component52, a power regulator component 54, and a microprocessor 56 incommunication with the layered heater 12. The microprocessor 56 is alsoin communication with a communications component 58, where certainoutput from the heater system 10 (e.g., temperature readings) isdelivered and also where input (e.g., updated TCR values, calibrationdata, temperature set points, resistance set points) may be provided tothe heater system 10.

Referring now to FIG. 6, the voltage measurement component 52 of thetwo-wire controller 14 is illustrated in greater detail. Generally, thetwo-wire controller 14 applies a DC bias, or low level DC current, tothe layered heater 12 during an AC power cycle zero-cross interval sothat the current value times a nominal heater resistance results in avoltage that is higher than the full wave voltage at the zero crossingfor a time period on each side of the zero value. During the timeinterval, the voltage of the layered heater 12 is amplified and comparedto a reference voltage, and power to the layered heater 12 is thencontrolled as further described herein. Application of the DC bias isfurther shown and described in U.S. Pat. No. 4,736,091, which iscommonly assigned with the present application and the contents of whichare incorporated by reference in their entirety. In another form of thepresent invention, an AC current may be used for the bias instead of theDC bias to determine the resistance of the layered heater 12.

As shown, the two-wire controller 14 comprises a transistor 60, a diode62, and a first resistor 64, wherein the first resistor 64 together withthe layered heater 12 form a voltage divider. For the DC bias, thetransistor 60 is turned on for a short time period, e.g., 200 μs, duringthe zero cross interval and further prevents current flow through thepower source 50 (not shown) during negative half cycles when the heateris receiving power. Additionally, the diode 62 prevents current flowthrough the power source 50 during positive half cycles when the layeredheater 12 is receiving power. The output of the layered heater 12 isthen sent through a second resistor 66 and into an opamp circuit 68 thatcomprises an amplifier 70 and resistors 72, 74, and 76. The outputvoltage of the amplifier 70 is thus used to calculate resistance anddetermine the temperature of the layered heater 12, wherein the outputvoltage of the amplifier 70 is read by an A/D converter within themicroprocessor 56. Further, during the DC bias time period, conversionof the output voltage of the amplifier 70 from an analog signal to adigital signal takes place, and a gating pulse from a triac 80 isdelivered to the layered heater 12 if the calculated resistance, orlayered heater 12 temperature, is such that a control algorithm hasdetermined a need for additional power from the layered heater 12. Asfurther shown, a field effect transistor 82 clamps the input of theamplifier 70, thereby preventing the amplifier 70 from being over drivenduring both positive and negative half cycles when the heater isreceiving line power.

The microprocessor 56, which is described in greater detail below,generally communicates with the circuit shown through an output control84, a bias control 86, and heater input 88. Additionally, themicroprocessor 56 further comprises firmware 90, and/or software (notshown). The firmware 90 may be programmed for a variety of functions,including but not limited to, allowing half cycle delivery of power toimprove controllability or full cycle power in accordance with IEEE 519.As a further example, the firmware 90 may include control algorithms tocompensate for thermal transient response and other calibration data aspreviously described. Therefore, the microprocessor 56 is used incombination with the DC bias circuitry to determine layered heater 12temperature and to more efficiently control power to the layered heater12.

A further expansion of the two-wire controller 14 is now shown ingreater detail in FIG. 7. The power source 50 is preferably non-isolatedand capacitively coupled with a linear regulator 100 as shown. The powersource 50 thus regulates an alternating current down to a specifiedvalue as required for operation. As further shown, the sine wave for thezero-cross (DC biasing) from the power source 50 is in communicationwith the microprocessor 56. During the zero-cross interval, the DC biasis applied through the transistor 102, diode 104, and resistor 106. Thevoltage across the layered heater 12 is amplified and offset by theamplifier 108, and the amplifier 110 is used as a reference for the A/Dconverter within the microprocessor 56 for temperature variances.

Measurement of the change in voltage across and current through thelayered heater 12 is accomplished using the dual amplifiers 112 and 114and analog switches 116 and 118, wherein the change in voltage signal isthrough amplifier 112 and analog switch 116, and the change in currentis through amplifier 114 and analog switch 118. As further shown, thechange in current is measured using a shunt resistor 1 16. Additionally,the two-wire controller 14 comprises a triac 120 that is out ofconduction at the zero-cross and is conducting on each half cycle.During the DC biasing interval, an A/D conversion takes place and thetriac 120 delivers a pulse if the measured resistance is such that thecontrol algorithm has determined a need for additional power from thelayered heater 12. Therefore, two methods of calculating resistance areprovided by the circuit shown in FIG. 7, namely, the DC bias circuit andthe shunt resistor circuit. Additionally, although the present inventionpreferably measures voltage and current to determine resistance,alternate methods of determining resistance such as a voltage gate orusing a known current may also be employed while remaining within thescope of the present invention.

In yet another form, the triac 120 is preferably a random fire triacsuch that the layered heater 12 is fired at high conduction angles toreduce the amount of energy that is delivered to the layered heater 12during sampling. For example, firing the layered heater 12 at conductionangles of 160° and 340° allows for sufficient sampling at 120 Hz withreduced energy input to the layered heater 12. Alternately, sampling atonly 160° or only 340° would result in a sampling rate of 60 Hz whilereducing the energy input further in half. Additionally, when using arandom fire triac, any rate function may be applied by delivering energyin smaller increments as the temperature (or resistance in another form)approaches the set point. Accordingly, the layered heater 12 is fired athigher and higher conduction angles into a full line cycle.

As further shown, communications to and from the two-wire controller 14take place on the opposite side of the microprocessor 56. Thecommunications component 58 comprises a series of opto-isolators 122,124, and 126, in addition to a line transceiver 128. Therefore,communications can be made through any number of protocols, including byway of example, RS-485 communications as illustrated herein. In additionto other functions, calibration data can be entered utilizing thiscommunications interface.

The firmware 90 is loaded into the microprocessor 56 using the ISP(In-System Programming) connections as shown. Therefore, certainmodifications to the settings within the two-wire controller 14,including entry of calibration data as previously described, can beaccomplished in an efficient manner.

The specific circuit components, along with the values and configurationof the circuit components, (e.g., resistor values, capacitor values,among others), as detailed in FIG. 7 are exemplary of one form of thetwo-wire controller 14 and should not be construed as limiting the scopeof the present invention. Accordingly, alternate circuit components,configurations, and values, and resistance measuring circuit topologiesmay be implemented in a two-wire configuration as defined herein whileremaining within the scope of the present invention.

Hot Runner Nozzle Application

One known application for the heater system 10 according to theprinciples of the present invention is for hot runner nozzles ininjection molding systems as shown in FIG. 8. The hot runner nozzles 150are typically disposed within a hot runner mold system 152, whichfurther comprises a plurality of mold wiring channels 154 that providefor routing of electrical leads (not shown) that run from heaters (notshown) disposed proximate the hot runner nozzles 150 to a two-wirecontroller (not shown) as described herein. Since each heater serves asboth a heating element and as a temperature sensor, only one set ofleads per heater is required rather than one set of leads for the heaterand one set of leads for a temperature sensor. As a result, the amountof leads running through the mold wiring channels 154 is reduced in halfand the related bulk and complexity is drastically reduced.

Additionally, injection molding equipment typically includes anumbilical 164 that runs from the controller to the hot runner moldsystem 152, wherein all of the leads and other related electricalcomponents are disposed. With the drastic reduction in the number ofleads provided by the present invention, the size and bulk of theumbilical 164 is also drastically reduced. Moreover, since thetemperature is being sensed by the entire resistive layer of the heater,the temperature is being sensed over a length rather than at a pointwith a conventional thermocouple.

Referring now to FIGS. 9 and 10, the heater system for a hot runnernozzle 150′ is illustrated in greater detail. The heater system 200comprises a layered heater 202 disposed around a body 203 of the hotrunner nozzle 150′, and a two-wire controller 204 in communication withthe layered heater 202 through a single set of leads 205. The layeredheater 202 further comprises a substrate 206, which is configured to fitaround the geometry of the hot runner nozzle 150′ (shown ascylindrical). The layered heater 202 further comprises a dielectriclayer 208 disposed on the substrate 206, a resistive layer 210 disposedon the dielectric layer 208, and a protective layer 214 disposed on theresistive layer 210. As further shown, terminal pads 216 are disposed onthe dielectric layer 208 and are in contact with the resistive layer210. Accordingly, the electrical leads 205 are in contact with theterminal pads 216 and connect the resistive layer 210 to the two-wirecontroller 204. As a result, only one set of electrical leads 205 arerequired for the heater system 200, rather than one set for the layeredheater 202 and another set for a separate temperature sensor.

As shown in FIG. 11, in an alternate form a layered heater 202′ isdisposed on an outer surface 220 of the hot runner nozzle 150′ ratherthan on a separate substrate as previously described. Similarly, thelayered heater 202′ comprises a dielectric layer 208′ disposed on theouter surface 220, a resistive layer 210′ disposed on the dielectriclayer 208′, and a protective layer 214′ disposed on the resistive layer210′. Terminal pads 216′ are similarly disposed on the dielectric layer208′ and are in contact with the resistive layer 210′. As further shown,the single set of leads 205′ connect the heater 202′ to the two-wirecontroller 204′.

In yet another form of the present invention, a modular solution toretrofitting the heater system according to the present invention withexisting controllers that use separate temperature sensors, e.g.,thermocouples, RTDs, thermistors, is provided and illustrated in FIG.12. As shown, two-wire modules 230 are provided between layered heaters232 and an existing temperature controller 234. The temperaturecontroller 234 comprises temperature sensor inputs 236 and power outputs238. The two-wire modules 230 thus contain the two-wire resistancemeasuring circuit as previously described, and the temperaturescalculated within the two-wire modules 230 are transmitted to thetemperature sensor inputs 236 of the existing temperature controller234. Based on these temperature inputs, the temperature controller 234controls the layered heaters 232 through the power outputs 238. Itshould be understood that power control may be a part of the temperaturecontroller 234 or may be a separate power controller 240 as shown whileremaining within the scope of the present invention. Accordingly,existing temperature controllers can be retrofitted with the two-wiremodules 230 to implement the heater system of the present inventionwithout substantial rework and modification of existing systems.

Referring now to FIG. 13, another form of a heater system according thepresent invention that reduces the number of electrical leads isillustrated and generally indicated by reference numeral 300. The heatersystem 300 comprises a layered heater 302 and a controller 304 thatoperate as previously described wherein a resistive layer (not shown) ofthe layered heater 302 is both a heating element and a temperaturesensor. The heater system 300 further comprises a power source 306,which is preferably low voltage in one form of the present invention,that provides power to the layered heater 302. The layered heater 302 isconnected to the controller 304 as shown through a single electricallead 308 and through the body or structure of a device 310 (e.g., hotrunner nozzle system mold) designated as a common return or neutral,wherein the common return device 310 provides an electrical return tothe controller 304 from the layered heater 302. The heater system 300uses the electrically conductive nature of the device 310 materials tocomplete the electrical circuit, and thus a power source 306 is requiredto limit the current level traveling through the device 310. Therefore,since the device structure 310 is being used to connect the layeredheater 302 to the controller 304, another electrical lead is eliminatedsuch that the controller 304 is effectively a “single-wire controller.”

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. A heater system comprising: a substrate disposed proximate a part tobe heated; a layered heater disposed proximate the substrate, thelayered heater comprising: at least one dielectric layer; at least oneresistive layer disposed on the dielectric layer, the resistive layerhaving sufficient temperature coefficient of resistance characteristicssuch that the resistive layer is a heater element and a temperaturesensor; two electrical lead wires connected to the resistive layer; anda two-wire controller connected to the resistive layer through the twoelectrical lead wires, wherein the two-wire controller determinestemperature of the layered heater using the resistance of the resistivelayer and controls heater temperature accordingly through the twoelectrical lead wires, wherein the heater system provides heat to thepart to be heated.
 2. The heater system according to claim 1, whereinthe two-wire controller comprises a DC bias control for calculation ofthe resistance of the resistive layer.
 3. The heater system according toclaim 1, wherein the two-wire controller comprises an AC bias controlfor calculation of the resistance of the resistive layer.
 4. The heatersystem according to claim 1, wherein the two-wire controller compriseshigh conduction angle firing.
 5. The heater system according to claim 1,wherein the two-wire controller comprises a shunt resistor forcalculation of the resistance of the resistive layer.
 6. The heatersystem according to claim 1, wherein the two-wire controller furthercomprises a microprocessor.
 7. The heater system according to claim 1,wherein the two-wire controller further comprises firmware.
 8. A methodof operating a layered heater comprising the steps of: placing asubstrate proximate a part to be heated; supplying power to the layeredheater through two electrical lead wires to a resistive layer of thelayered heater; transferring heat from the resistive layer, through adielectric layer of the layered heater, and to the substrate; andcalculating the temperature of the resistive layer using a two-wirecontroller connected to the layered heater through the two electricallead wires, wherein the resistive layer is a heater element and atemperature sensor.
 9. The method according to claim 8, furthercomprising the step of resistance data calibration.
 10. The methodaccording to claim 8, further comprising the step of lead wirecalibration.
 11. The method according to claim 8, further comprising thestep of temperature calibration.
 12. The method according to claim 8,further comprising the step of TCR calibration.
 13. The heater systemaccording to claim 1, wherein the layered heater is selected from agroup consisting of thick film, thin film, thermal spray, and sol-gel.